To obtain hydrocarbons such as oil and gas from subterranean formations, wellbores are drilled into the subterranean formations by rotating a drill bit attached to an end of a drill string. A substantial portion of current drilling activity involves what is referred to in the art as “directional” drilling. Directional drilling involves drilling deviated and/or horizontal wellbores. Modern directional drilling systems generally employ a bottom hole assembly (BHA) at the end of the drill string that includes a drill bit and a hydraulically actuated motor to drive rotation of the drill bit. The drill bit is coupled to a drive shaft of the motor, typically through an assembly configured for steering the path of the drill bit, and drilling fluid pumped through the motor (and to the drill bit) from the surface drives rotation of the drive shaft to which the drill bit is attached. Such hydraulic motors are commonly referred to in the drilling industry as “mud motors,” “drilling motors,” and “Moineau motors.” Such motors are referred to hereinafter as “hydraulic drilling motors.”
Hydraulic drilling motors include a power section that contains a stator and a rotor disposed in the stator. The stator may include a metal housing that is lined inside with a helically contoured or lobed elastomeric material. The rotor is usually made from a suitable metal, such as steel, and has an outer lobed surface. Pressurized drilling fluid (commonly referred to as “drilling mud”) is pumped into a progressive cavity formed between the rotor and the stator lobes. The force of the pressurized fluid pumped into and through the cavity causes the rotor to turn in a planetary-type motion. A suitable shaft and a flexible coupling compensate for eccentric movement of the rotor. The shaft is coupled to a bearing assembly having a drive shaft (also referred to as a “drive sub”), which rotates the drill bit through the aforementioned steering assembly.
As drilling fluid flows through the progressive cavity between the rotor and the stator, forces on the rotor and the stator, as well as abrasives in the drilling fluid, can damage parts of the motor. The motor may include a resilient portion (e.g., an elastomeric or rubber portion), typically as part of the stator, which is designed to wear. The elastomeric portion may be replaced after a certain amount of use, or when a selected amount of wear or damage is detected.
The resilient portion typically swells under conditions encountered in drilling operations, such as due to chemical interaction with drilling fluids, thermal effects, or other factors. Such swelling changes the spacing and fit of the rotor with the stator. Metal parts of the motor may also expand with temperature, further changing the spacing and fit of the rotor with the stator. Hydraulic drilling motors may typically be made with rotors slightly undersized to allow room for the resilient portion of the stator to swell, and the motors may be operated at relatively lower pressures and lower power until the resilient portion of the motor swells enough to form a seal at full operating pressure. For example, a motor may be operated for thirty (30) to sixty (60) minutes at low-pressure, break-in conditions, before operation at the motor's design conditions. Downhole pressure may counteract some of the swelling effect by compressing the resilient portion of the motor.